542 Geological Structure of the Sahara. [September, 
which follow, further to the south, sandstones, palaeozoic 
slates, sometimes interpenetrated by granite and porphyry, 
as also quartzite and azoic clay-slate. 
In the depression between the Atlas and the Ahaggar 
Mountains middle and upper cretaceous rocks form the sub- 
stratum, whilst quaternary sandy fresh-water clays, with 
gypsum and rock-salt, constitute the superficial layers. 
The same cretaceous deposits form the soil of the Hammada 
el Homra, and of the Harudj Mountains in Tripoli. In the 
south it is diredtly followed by Devonian sandstone. The 
latter, with the underlying limestones and slates, is the pre- 
dominant formation to the southern limit of the Desert. 
Permian, triassic, jurassic, and subcretaceous formations 
have been hitherto detected neither in the Sahara nor the 
Egyptian frontier mountains. The great plateau-mountains 
of Ahaggar in Air and Tibetsi seem principally to consist of 
palaeozoic sandstone, clay-slate, gneiss, granite, and recent 
eruptive volcanic rocks. Tertiary deposits of marine origin 
are to be found only to the north of the Chotts of Tunis. 
They occur also to a considerable extent in the Libyan and 
Arabian Deserts. In the north-eastern Sahara and in Egypt 
the eocene nummulitic rocks extend southwards to the lati- 
tude of Esneh : the miocene rocks have their southern 
limit at the oasis of Sinah, and the hills between Cairo and 
Suez. 
The southern and a part of the central Sahara have been 
dry land since the end of the Devonian period ; the greater 
part of the remaining Sahara was left dry after the creta- 
ceous epoch. The sea still maintained itself in the Libyan 
Desert during the eocene, and in the northern part of that 
region down to the middle miocene. 
The eruptions of the basaltic, phonolithic, and trachytic 
rocks in Tripoli, the Libyan and Arabian Deserts, as also 
probably those in the mountain-lands of Ahaggar and Tubu, 
occasioned but little disturbance in the adjacent formations, 
and must have ensued chiefly in the later tertiary times. 
During the diluvial period the Sahara, as well as a part 
of the southern and eastern Mediterranean, was dry land. 
The hypothesis of a diluvial Sahara sea is confirmed neither 
by the geological structure nor the surface appearance of the 
Desert. At the utmost the region of the Tunisian Chotts 
may have been connected with the Mediterranean, and per- 
haps the narrow depression between Alexandria and the 
Ammon oasis with the Red Sea. 
During the Diluvial period there prevailed in North Africa 
